Satellite images have revealed the cause of the flooding in Nepal: part of a mountain collapsed along with a glacier

According to the GFZ Helmholtz Centre for Geosciences, it was not just a glacier that broke away from the slopes of the Langtang massif – a mass of rock came crashing down with it. A huge mass of ice and rocks surged down into the valley and turned into a fast-moving mudslide.

Details

Seismometers recorded the collapse at 02:52 UTC. The signal was so powerful that the event was initially identified as an earthquake with a magnitude of around 5.7. The seismic waves reached the GFZ seismic stations in Germany in approximately 40 minutes. In the GEOFON catalogue, the event is listed as a landslide near the Nepal–China border.

According to scientists’ assessments, a hanging glacier broke away from the northern slope of the Langtang massif, along with the bedrock beneath it. The ice and rocks tumbled down the steep slope. As they moved, a large amount of heat was likely released, causing significant volumes of ice to melt almost instantly.

The water mixed with boulders, debris and sedimentary rock to form a powerful mudslide, which surged down the Lende Khola. According to GFZ calculations, its speed reached 40–50 metres per second — over 130 km/h — and the height of the mud-water wave was several tens of metres. In less than ten minutes, the flow reached the Gyirong border post and destroyed the infrastructure there.

The water and debris then continued down the Trisuli River valley. The area of destruction stretched for more than 100 kilometres, and at the confluence with the Seti Gandaki, the water level rose by more than 10 metres. The flood spread far beyond the high-altitude regions of the Himalayas.

The extent of the loss of life is still being ascertained. According to the Nepalese authorities as of the morning of 31 August, the bodies of 903 people have been recovered, whilst a further 4,247 are listed as missing. Among those missing are hundreds of foreign nationals and workers at hydropower facilities.

If the Chinese side of the border is also taken into account, the latest published figures put the death toll at 919 and the number of people missing at 4,793 in Nepal and Tibet. Rescue operations are continuing, including in the tunnels of hydroelectric power stations, where rescuers are attempting to reach the site through a layer of mud and debris.

Why this matters

Research by the GFZ shows that catastrophic floods in the Himalayas can be triggered not only by glacial lake outbursts or heavy rainfall. A mountainside collapse, together with a glacier, can create a torrent within minutes that travels tens of kilometres, leaving virtually no time for evacuation.

It is impossible to prevent the collapse itself, the GFZ notes, but scientists consider early-warning systems based on seismometers to be promising. These systems are capable of detecting such large-scale movements almost in real time and, in theory, giving people downstream at least a few extra minutes to evacuate. The GFZ is already developing a system that combines seismic data, river and weather observations, and satellite imagery.

Source

Main source — GFZ Helmholtz Centre for Geosciences, the article ‘Extreme flooding in China and Nepal’ dated 27 August 2026, and the GEOFON catalogue of seismic events. Current figures on casualties – Nepal’s National Disaster Risk Reduction and Management Authority, published on 31 August